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Advisor

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Second Committee Member

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Abstract

Almost 20 years after the remarkable earthquake swarm of 1982, near the town of Enola, Arkansas, with more than 40,000 micro-earthquakes, another event revisited the same North-Central Arkansas region in 2001. Nine years later, in 2010, a huge swarm event shook the northern part of Faulkner County, around the city of Guy. The following year, this seismic swarm event apparently migrated southward towards the city of Greenbrier, with an increase in the number of yearly recorded events. A 13km previously unrecognized, NE trending Guy-Greenbrier basement fault was revealed as a result of these swarm events.

Within the vicinity of the Greenbrier earthquake swarm in the eastern Arkoma basin, north of the Ouachita Mountains, the first waste water disposal well became operational in April 2009, and a total of six disposal wells are known to have been operating between 2009 and 2011. The deepest of these was the Wayne Edgmon well, which was injecting above the intersection of the Enders and the Guy-Greenbrier fault. The area experienced an increase in magnitude M ≥2.5 earthquakes during periods of Saltwater Disposal (SWD). The Enola and Greenbrier swarms are known to be in an intraplate setting and the generation of earthquakes in such a setting is uncommon. In the Enola swarm vicinity, a leveling survey in 1986 revealed a rise in measured elevation within the Paleozoic graben, where the swarm hypocenters are located, and correlated as a possible cause.

The Enola sequence still has unanswered questions and the Guy-Greenbrier swarm raises the possibility to find answers to these questions. Though both swarms are tectonically and geologically related, they do not seem to have similar triggering mechanisms as there were no disposal wells in the study area prior to 1982. The recent NGS levelling survey showed insignificant changes in elevation, thus graben uplift is still considered a triggering mechanism for the Enola swarm. With the onset of SWD wells and increase in seismicity between the towns of Guy and Greenbrier during the same time, as well as responses to the seismic profiling questions in regards to background seismicity, injection practices, temporal and spatial correlations, the Greenbrier appears to be an induced event, while the Enola event appears to be unrelated to human activity.